Photographic film scanners are known in which image frames on a strip of photographic film are scanned to convert the optical image frames into digital image data which can be stored and manipulated in a computer and, at the user's option, sent to a digital printer for generating hardcopy image prints. It is common in such scanners, to scan the image frame by transporting the film strip through an optical imaging path in which an exposure light source is shone through the image and focussed onto a linear CCD sensor to capture the image information one scan line at a time. The exposure light source may comprise a linear, interleaved array of spectrally distinct light emitting diodes (LED), for example, emitting in the red, blue and green spectra. In order to achieve accurate color rendition in the scanner it is necessary to perform a calibration of the LED exposure lights primarily to equalize the light outputs of the respective red, blue and green LED's. The analog pixel-by-pixel image information captured on the linear CCD light sensor must be clocked out of the CCD, digitized, processed and transferred to a host computer in synchronism with the transport motion of the film in such a manner that the transferred data is accurate and that no data is lost as a result of asynchronous operation between the scanner and the host computer. Internal operations of the scanner typically involve the use of separate timers to control the light source, film transport motor drive and data clocking/transfer functions, which require complex synchronizing provisions for control of the separate timers. Changing the scanning resolution complicates the control procedure as does changing the operating conditions of the R,B,G exposure light sources to accommodate for system drift and differing density characteristics among different film strips.
In the above cross-referenced copending U.S. application Ser. No. (Dkt 77989), a photographic film scanner is described that employs a programmed controller which operates in a manner to address the concerns just described. The controller is provided with a common timer and is programmed with a plurality of lookup tables (LUT), each LUT being populated with a sequence of elements defining timing of an operating activity of a respective one of the illuminant head light sources, the stepper motor and the light sensor data transfer circuits. The program operates to use the common timer to step synchronously through the elements of the LUT's to output value states from the elements of each of the LUT's; so as to suitably control the actuation timing of the respective light sources, stepper motor and data collection and transfer circuits. The arrangement described is a simple and convenient method and apparatus for control timing of critical operating functions in an image scanner. The use of a plurality of lookup tables provides flexible, independent control of the integration periods of the three different color planes.
The host computer operates in an asynchronous manner relative to the timing operation of the film scanner. The host computer can have other tasks to perform that need to be completed before it can accept data from the scanner. In the scanner operation described above, however, the scanner is integrating light from the LEDs and if the host is late in accepting data from the scanner, and incorrect amount of light will have been collected and a bright line will ultimately appear in the reproduced image. There is therefore a need in the scanner of the type described to accommodate asynchronous operation of the host computer so that delays in accepting data from the scanner do not adversely affect the image data being transferred and thus do not create artifacts in the image reproduced from the transferred data.